A vertical or trench-type metal-oxide-semiconductor (hereinafter referred to as MOS) field-effect transistor is known which has a deep trench in a silicon substrate with a thin gate oxide film formed on a sidewall of the trench. The substrate is formed with drain regions located across the trench and spaced apart along the sidewall from the source region formed by the substrate so that a channel region vertically extends between the source region and each of the drain regions. As well known in the art, the threshold characteristics of such a trench transistor are controlled by varying the doping level of the substrate throughout the horizontal area of the layer spanning between the source and drain regions. Such doping techniques are contrasted by an ordinary channel doping process used for the fabrication of planar MOS devices in that the dopant atoms to be injected into the target are bombarded in directions generally parallel with the sidewall of the trench in the substrate. Difficulties are therefore encountered in controlling the impurity distribution profile selectively for the channel region alone of the trench transistor, thus requiring a tradeoff of varying the doping density throughout the horizontal area of the layer between the source and drain regions. When the gate oxide film is grown on the sidewall of the trench in the substrate thus doped throughout, the impurities which have been injected into the substrate tend to decrease in an area close to the trench sidewall due to outdiffusion induced by the thermal oxidation process. To compensate for such localized reduction in the density of impurities in the sidewall region of the substrate, it is required to have the substrate doped to a density higher than the desired doping level for the channel region which is to be formed by the sidewall region of the substrate. The use of an increased substrate doping level however causes various problems such as the deterioration of the dynamic and breakdown-voltage characteristics of the trench-type MOS field-effect transistor device.
It is, accordingly, an important object of the present invention to provide an improved insulated-gate field-effect transistor device which is easy for the control of the threshold voltage and is reliable in performance.
It is another important object of the present invention to provide a process of fabricating such an improved insulated-gate field-effect transistor device.